The invention relates to an apparatus and a method capable of ascertaining and displaying deformations or shifts in test objects in real time.
In some measurement tasks, the deformation or shifting, particularly of diffusely scattering object surfaces, must be detected precisely, for instance to examine the influence of an exertion of force on a test object. There is often a need to be able to follow the deformation of the test object or its surface directly, while the surface structure or shape itself occasionally does not matter. This is especially pertinent to practical conditions, that is, close to production and with the deformation of the surface being displayed in the clearest, most readily apparent and readily evaluatable possible form.
For detecting surface displacements or expansions of a test object whose surface scatters diffusely, strike projection methods, such as the moirxc3xa9 method, and interferometric methods are known, such as electronic speckle pattern interferometry (ESPI method) or the shearing method. While the moirxc3xa9 method is more useful for larger deformations or shifts, the interferometric methods are used particularly for measuring lesser travel distances but with greater resolution. There are no restrictions to object size in the speckle method.
From U.S. Pat. No. 4,660,978, a shearing interferometer with a fixedly adjusted mirror tilt is known for determining distortions of a wave front of an optical beam. The shearing interferometer has a beam splitter that splits the arising beam, which possibly has a curved wave front, into two split beams and reunites them. The two split beams are each reflected by mirrors. One of the mirrors is connected to an inclination device, while the other mirror is connected to a displacement device. The split beams reunited by the beam splitter are carried to a camera. For different mirror inclinations of the inclinable mirror, the displaceable mirror is then adjusted incrementally. From the brightness or intensity values that result at the camera, the phase relationship of the beam can be determined, and from this data pertaining to the curvature of the wave front can be calculated.
Similar devices can be used to examine deformations of object surfaces. For instance, two states of a test object are compared for the purpose, in which the object is picked up in two different load states and the interferograms of the two states are subtracted. The result is a differential interferogram, which, depending on the measurement principle employed, shows either the displacement or the expansion of the object between the two states in the form of lines of interference. The amount of displacement or expansion at a pixel of the differential interferogram can then be determined, for instance by counting the interference lines beginning at a pixel of known displacement or expansion and taking the wavelength of the light employed into account.
If the measuring head, in a similar way to the shearing method described above, is equipped with a phase shift unit, then an expanded evaluation can be performed by the principle of the phase shift method (W. Osten, xe2x80x9cDigitale Verarbeitung und Auswertung von Interferenzbildernxe2x80x9d [Digital Processing and Evaluation of Interference Patterns], Chapter 6, Akademieverlag, ISBN 3-05-501294-1). In it, phase images are generated that assign a certain phase angle to each pixel. If the phase images of two states of the object are subtracted, the result is a phase differential image. In contrast to the aforementioned differential interferogram, the phase differential image does not have sinusoidally modulated interference lines but instead directly indicates the phase difference angle between the two states of the object.
In the phase shift method, the object to be examined, if successive images of the same object""s state are to be picked up in succession with a different phase relationship, must remain absolutely still.
To aid in this, German Patent DE 38 43 396 C1 discloses a method known as xe2x80x9cdirect phase measurementxe2x80x9d or as the xe2x80x9cspatial phase shift methodxe2x80x9d. All that this method needs for calculating 2xcfx80-modulated phase images is a grating projection and a camera image.
It is an assumption here that the period length in the striped pattern corresponds to a constant number of pixels, and that the background intensity of adjacent pixels is identical. This represents a rough approximation of actual conditions and leads to phase errors.
In testing technical objects, it is important to facilitate the evaluation of differential interferograms, so that defects in an object or other special features of the test object that can be detected by the deformation can be made clearly apparent. U.S. Pat. No. 5,091,776, teaches reconfiguring an obtained differential image before displaying it, or in other words, finding the amount of the difference pixel by pixel. The absolute value of the difference generated can furthermore be multiplied by a constant factor via an amplifier, and thus the overall image contrast can be varied.
Locally fluctuating light conditions can lead here to restricted evaluatability of the images obtained.
With this as the point of departure, it is the object of the invention to create an apparatus and a method capable of real-time ascertainment and display of deformations of test objects that furnish an improved image quality. This object is attained by the method and measuring system of the present invention.
In the method of the invention, in a first method step, the initial step, a normalization data set is generated from a set of phase-shifted images of the surface region of interest on the object; this data set contains normalization information specific for each pixel. This normalization data set is specific for the region of a object surface that is located in the viewing field of a picture taking device. The normalization data set is stored in memory and held in readiness for using the further post-treatment of the images taken of the same surface region. For each pixel, the normalization data set contains information about the amplitude with which the image brightness or intensity varies if a phase displacement is performed. Thus the normalization data set represents an amplification or normalization factor that is specific for each pixel. By applying this normalization data set to the differential images generated, an image suitable for display on a monitor is obtained that has equally good contrast at every detail of the image.
The method is suitable for measuring object deformations both by the stripe method and by speckle modulation, for instance. In both cases, a marked improvement in image quality compared to an uncorrected method is obtained.
In order to display a deformation of a test object compared with an initial state, it is expedient to store a reference image in memory that is utilized together with an image picked up currently for generating a differential image. It is possible to pick up moving object surfaces. If the measurement object moves during the period required by a camera to read out its image sensor, this does not fundamentally impede the picture taking. It is therefore possible to observe the test object continuously with the camera; the difference between the currently arriving image and the reference image stored in memory is formed pixel by pixel. These differential values calculated pixel by pixel are normalized using the applicable pixel-specific amplitude value of the stripe or speckle modulation and are displayed as a normalized differential value image.
In many cases, it is expedient to generate a quantitative value image pixel by pixel from the differential image. This improves the display. The zero crossover of the sinusoidal course of intensity of a speckle becomes black, and both positive and negative values are represented by gray values.
To determine an image to be displayed of the current state of the test object, only a current camera image and the reference image stored in memory in the computer, along with the normalization data set, are needed. The method is therefore capable of working in real-time; that is, pictures taken by the camera can be processed immediately and shown on the monitor. A suitably powerful computer system makes high image refresh rates possible.
Normalizing the gray values of the resultant image pixel by pixel makes the image measurable; that is, measurements can be taken in the resultant image, for instance, to determine structural defects. The image can also be displayed as a color picture. By eliminating the dependency of the resulting value DN(x,y) from the intensity amplitude IA(x,y), each automatic evaluation of the corrected pattern of interference lines is furthermore facilitated substantially, since now it may be assumed that
DN(x,y)=k sin "PHgr"(x,y),
with a constant, location-independent value k for all pixels
To generate a normalization data set, that is, before the onset of reproduction of the motion of the object surface in the interference pattern or by means of stripe projection, preferably at least three, and preferably four, interference patterns or stripelike projection patterns of different phase relationships are picked up. To that end, a picture-taking device is, for instance, used that has a light source for lighting the object""s surface with coherent light. Diffuse light backscattered from the object surface is picked up, for instance, with an interferometer, which splits the light detected by way of a lens into two split beams, which are then reunited and carried to a camera. One of the split beams is adjusted in a defined manner, for instance, by purposeful motion of a mirror with a suitable actuator, like a piezoelectric drive, so that a phase displacement between the split beams occurs. For instance, four images, each with a phase relationship offset by xcfx80/2, can be picked up. For each pixel, four brightness or intensity values are thus present, from which the brightness amplitude IA for the applicable pixel can be determined. Only for this provision, which is part of the preparatory step, does the object""s surface have to remain still. Once an image that has been taken is stored in memory as a reference image and the normalization data set is determined, the object""s surface can be moved somewhat. To generate a stripelike pattern that characterizes the motion of the object""s surface, the new speckle pattern obtained is subtracted point by point from the reference image and, after optional quantification, divided point by point by the pixel-specific amplitude value. The order in which the normalization and subtraction or amount finding are done does not matter. It can also be reversed.
Once the individual images have been obtained from the picture taking unit by the speckle measuring technique, individual pixels can occur whose brightness value is phase-independent in the initial step. For these pixels, in an advantageous embodiment of the method and the measuring system, substitute values can be displayed. These substitute values can be formed from the neighboring points, for instance by averaging. It can also be advantageous for the resultant image, obtained by subtraction and normalization, to be smoothed with a low-pass filter. In this way, image noise can be reduced. It is furthermore possible for the resultant image to be scaled to the gray range that can be shown on a monitor.
Advantageous details of embodiments of the invention will become apparent from the drawings, the description, and the claims.